Project Summary/Abstract Tendons connect muscles to bones and transmit the force generated by muscle contraction. Because of their unique position, the tendons are subject to significant shear and compression forces resulting in frequent tendon damage and tears. Tendons are also highly susceptive to development of degenerative conditions and because of slow and limited healing capacity tendon and ligament conditions account to more than 50% of visits to orthopedic clinics and present a major burden to individuals and society. A relative increase in tendon and ligament conditions in recent years led to a significant increase in the number of studies and research grants associated with tendons. This research is largely focused on clinical applications in tendons and it was recently recognized at NIAMS and within the research community that the basic research in tendons remained too limited and the absence of basic information resources is a major impediment to progress in tendon research. This understanding was indeed emphasized in the conclusions of a Tendon related conference sponsored by IAMS a few years ago. However, because of internal NIAMS policies, this realization was not followed by an RFA or NIH driven research consortium to address these experimental needs. Our labs has been at the forefront of identifying new tendon genes and developing research tools for tendon research in mouse and sharing them with the tendon community. We therefore decided to propose a project focused and the development of experimental resources and the rapid sharing of these resorces. The first aim in this project is therefore to use RNAseq in various developmental and mature stages and by comparison identify tendon differentiation stages and develop for each of these stages a comprehensive and authoritative list of genes with distinctive expression in tendons. In addition we will define a prorotypic short least of genes whose expression define these stages and can be used as success criteria for the programing of stem/progenitor cells to the tendon cell fate stages. In the second specific aim we will perform single cell RNAseq transcriptome assays to identify potential diversity of sub-specialized tenocytes in tendons. One cell type we will focus on in this aim is the epitenon, a critical part of the tendons that receives very limited research attention to date. Finally, the 3rd specific aim is to develop new mouse models for tendon research including additional cre lines with more specific targeted activity in tenocytes and associated cell types. And to develop additional mouse mutants to identify genes regulating the tendon cell fate. Since this project is defined as a resource development grant we will further focus on the rapid dissemination of all these data resources and mouse lines to support a more rapid progress in tendon and ligament research.